A fire alarm system typically includes one or more notification appliances that notify the public of an alarm. A Notification Appliance Circuit (NAC) powers the notification appliances that are connected to a fire alarm control panel. A primary power source (such as line power from an AC line) may supply power to the fire alarm control panel. The fire alarm system may also include a backup voltage source that supplies power to the fire alarm control panel. The backup voltage source (such as a battery) is used when the primary power source is unavailable. Abnormal conditions may cause either the primary or backup power supply to operate at a voltage less than nominal. The lowest voltage that will power the NAC is defined as the worst case operating voltage. The NAC may provide power from the control panel to the notification appliances. The notification appliances draw a significant amount of current from the NAC and create a voltage drop across the wires. The voltage drop may reduce the voltage supplied to the notification appliances at the end of the NAC (opposite the control panel) to a level that is below the voltage necessary to power the notification appliance.
Notification Appliances have a specified operating range. During the design of the fire alarm system, a designer estimates whether all the notification appliances will be powered above their specified minimum operating voltage at the worst case operating voltage. To make this estimation, the designer predicts the voltage drop from the fire alarm panel to the last notification device. The voltage drop calculation is based on the electrical characteristics of the NAC as it is configured in the specific installation. The designer then subtracts the predicted voltage drop from the worst case output voltage of the fire alarm panel and compares the result to the minimum operating voltage of the notification appliance. The NAC design is acceptable when the calculated voltage is above the minimum operating voltage of the notification appliance.
However, the installed system may differ from the designed system. For example, the wiring distance of the NAC may differ due to practical considerations in the building, or alternate routings of the wires by the electrical installers. The actual voltage drop on a NAC in the installed system is frequently different than the calculated voltage drop. Therefore, it is important to confirm, after installation, that the NAC has sufficient voltage to operate the notification appliances.
Conventionally, it was difficult to test the voltage drop in an installed system. It was even more difficult to test the voltage drop at or near the lowest suitable voltage on the NAC. The lowest suitable voltage on the NAC is typically the voltage supplied from the control panel when the backup power source, for example, one or more batteries, are at the end of their rated life. The NAC voltage drop is difficult to determine at the lowest suitable voltage because the nominal output voltage of the control panel is significantly higher than the worst case operating voltage.
Notification appliances draw more current at low voltage than they do at higher voltages. If less current is drawn from the NAC, then the voltage drop across the NAC will also be reduced. Measuring the voltage at the control panel and then at the last notification appliance during higher voltage operation (supplied by the primary power source or the backup power source at the beginning of its rated life), will not give an accurate measurement of the voltage drop in the system during the lowest voltage operation (i.e. when the battery is at the end of its rated life).
In a system where the lowest voltage condition occurs when the batteries are nearly discharged, the only way to measure the voltage drop on a NAC during the lowest voltage operation and verify that it is within its designed parameters, is to power the system from batteries for an extended period of time, until the batteries are near their rated end of life and then activate the notification appliances and measure the voltage drop on each NAC. This is generally not practical and is often not done because it is time consuming and potentially damaging to the batteries. In a system where the lowest voltage occurs when the AC power supply is operating under abnormal conditions (for example a fault on the AC line lowers the system voltage), it is difficult to create the abnormal condition. It requires powering the panel from expensive equipment to vary the AC input. This equipment may be practical in a lab environment but very impractical for a field technician to carry. Accordingly, a need exists for testing whether the NAC is capable of operating at a reduced or worst case system voltage that is simple in design and operation.